This invention relates to an enclosed switch contact assembly and particularly to such a contact assembly forming an integrated component of an electric motor.
Switched electric motors operate in many different environments and often require ventilation to maintain long satisfactory life. The motors may operate in various adverse environments such as airborne contaminants including chemical substances, foreign particles and the like. Other motors are operated in high humidity environments. Switch assemblies used in the various contaminated environments may be subject to rapid deterioration as a result of the foreign matter in the environment.
Typically, a motor used to drive power equipment such as a table saw or the like operates in a highly contaminated environment. The fine sawdust for example will be carried by the air into the motor structure. A clothes dryer operates in an environment of both fine lint and high humidity. In a gas fired dryer, a motor actuated switch assembly may include two sets of contacts. A first set of contacts controls the induction motor start circuit and a second set of contacts controls a gas solenoid for the gas burner. Other applications will encounter similar problems, and even though not directly associated with a source of contaminants, may be subject to particulate, moisture and other contaminants as the result of the general working environment.
The airborne contaminates are carried about the motor and through the motor's ventilation system. In certain applications, corrosive chemicals reactions with contaminants may result and in most applications the contaminant adversely affect the switching function. A widely used motor is of a single phase induction motor design wherein a switch assembly is mounted within the motor structure and completes a start winding circuit only during the starting of the motor. A suitable device such as a centrifugal actuator is provided for opening of switch contacts to disconnect the start winding circuit after the motor accelerates to an appropriate speed. Various centrifugally actuated switches have been commercially developed for holding the contacts closed in the start position and automatically moving them to an open position at a selected appropriate speed. A particularly satisfactory switch actuating assembly is disclosed in the U.S. Pat. No. 4,419,550 which issued to Gerald J. Monette on Dec. 6, 1983. The switch structure includes a multiple arm leaf spring member secured to a rigid mounting plate within the motor end frame. The spring member includes a cantilevered contact arm with a button contact in opposed relation to a small button contact on the plate. A centrifugal actuator on the motor shaft is coupled to the leaf spring mount unit and is operable to hold the contact closed in the initial start position. The centrifugal actuator retracts under centrifugal forces as the motor accelerates and releases the leaf spring contact which rapidly snaps to the open position thereby disconnecting of the motor circuit. The contacts in accordance with various open motor constructions are exposed within the motor housing. In environments carrying airborne contaminants, the foreign matter can significantly adversely effect the functioning of the switch assembly. The various contaminants and airborne particles may for example change and increase the contact resistance resulting in increased heating of the contacts, as well as accelerating oxidation of the contacts with corresponding changes in the conductivity characteristic. These, and other results depending upon the particular environment can often lead to premature switch malfunction and eventually to premature switch failure.
Various solutions have been suggested. A "totally enclosed motor" construction can be provided wherein the outer motor frame is essentially sealed such that airborne particulates, contaminants and the like are totally or significantly excluded from the motor. In such motors, the motor is essentially formed with an outer enclosure which prevents outside air and foreign matter carried thereby from entering and circulating within the motor enclosure. Such motors however are generally quite costly and form an alternative to opened drip-proof motor of the same horsepower rating. Drip-proof and open enclosure motor construction are significantly less costly. The drip-proof motor will normally have a protective covering which will protect the direct entry of contaminants or the like into the motor but otherwise function in much the same manner of an open motor which allows free movement of the airborne products into the motor. The open-construction of the motor more readily dissipates heat and can be constructed as a more compact motor. A totally enclosed motor generally requires an elongated rotor and core construction to dissipate the same heat at the same horsepower rating. Such additional length in the stator rotor and stator require additional steel in the core structures, copper in the winding structures and the like, and consequently adds significant material cost as well producing a larger motor.
Thus, it would be desirable to provide an effectively open motor structure for a switched motor such as the widely used single phase induction motors for use in such adverse environmental conditions.
As an alternate to the relative costly totally enclosed motors, the prior art has suggested the use of a drip-proof or open type of motor construction including a specially enclosed start switch assembly to protect the switch contacts from contamination and thereby permit use of the open motor in environments containing airborne contaminants and particles. For example, U.S. Pat. No. 2,816,199 which issued Dec. 10, 1957 discloses a starting switch assembly including a dustproof switch mechanism. In the latter patent, a fixed contact is mounted in opposed relation to a contact carried by a spring arm with a centrifugal actuator to position the movable contact between an open and closed position. An outer flexible cup-shaped enclosure is located between the mounting plate and the spring arm. The cup has a closed end abutting the mounting plate and projects outwardly with the outer open end secured to a spring arm. The side wall of the cup is provided with a slot to permit telescoping of the cup-shaped enclosure upwardly over the fixed contact and its supporting arm. The outer open end of the cup-shaped member includes projections which mate with openings in the spring arm for the movable contact and the lugs are forced through the openings in the member and secure the cup-shaped member to the contact spring arm. The cup-shaped member is formed of a flexible material such as a silicone rubber which collapses as the centrifugal actuator moves the leaf spring arm towards the fixed contact to close the switch. Another contact enclosure for a centrifugally actuated switch is illustrated in U.S. Pat. No. 3,015,011 which issued Dec. 26, 1961. In this instance, the switch contacts are again mounted to a fixed mounting plate and to a leaf spring arm, the upper end of which projects outwardly into the path of the centrifugal actuator. The contact enclosure provided includes a stepped cylindrical member which includes an outer cylindrical end fitted within a cup-shaped mounting member, which is connected to the spring arm. The opposite end of the enclosure includes a similar cylindrical end of a substantially lesser diameter and connected to the first cylindrical end by a lateral flat wall which is integral with the two ends to form the stepped enclosure. The second end is secured to the mounting plate encircling the fixed contact. The total assembly is described as being free of cement or the like to permit limited breathing while preventing movement of contaminants into the enclosure. In operation, the lateral flat wall joining the two radially spaced end mounting wall circular structure defines first and second junctions and pivot connection which permit the collapsing of the assembly at the two connections with the wall moving to a Z-shaped configuration.
Additional prior art teachings are disclosed in the prior art cited in such patents as well as other references of lesser significance set forth in the accompanying prior art disclosure statement submitted herewith. Various problems are associated with dust-proof contact enclosures particularly in the start switch assembly for induction motors. The motor switch assembly is subjected to a large number of cycles over the life of the motor, and conductive arcs are created during the opening and closing of the contacts. The material of the enclosure must maintain the original characteristic over the life of the switch assembly.
Notwithstanding the prior art teaching, there is a need for a low cost improved contact enclosure which minimizes the loading on the spring arm to permit essentially unrestricted movement of the movable contact while establishing a highly effective enclosure to protect the contacts from contaminants in the surrounding environment. The enclosure must be flexible and operate for many switching cycles in the arcing environments of motor start switch assemblies and the like.